1. Field of the Invention
The present invention relates to a method of detecting a touch event for a touch panel and related device, and more particularly, to a method for enhancing efficiency of detecting a touch event and related device.
2. Description of the Prior Art
Touch panels, which are usually combined with liquid crystal displays to form touch screens, are popular interfaces that allow people to control consumer electronics and equipment easily. Capacitive touch panels have higher sensitivity and are more durable than other types of touch panels, such as resistive touch panels, and have become a mainstream touch panel technology. Among capacitive touch panel technologies, projected capacitive touch panels have the most potential, because the projected capacitive touch panels can precisely detect the location of a touch event, and multi-touch functions can be implemented in the projected capacitive touch panels. A projected capacitive touch panel consists of intersecting Indium Tin Oxide (ITO) traces that act as row and column electrodes. A coupling node is formed at each intersection of a row trace and a column trace, and a capacitor is formed between the coupling node and a ground. When a user touches or approaches the coupling node, a body capacitance may be coupled to the capacitor at the coupling node. Therefore, a location of this touch event is determined by detecting which trace capacitance change occurs on.
Please refer to FIG. 1, which is a schematic diagram of a touch control device 10 according to the prior art. The touch control device 10 is used for controlling a projected capacitive touch panel 12 consisting of intersecting ITO traces. The touch control device 10 comprises an analog-to-digital (A/D) converter 10 and a microprocessor 102. The A/D converter 100 is coupled to the touch panel 12 and the microprocessor 102, and is utilized for scanning all traces in the touch panel 12 for determining at which trace the capacitance change occurs. In detail, the A/D converter 100 sequentially outputs a charge control signal, e.g. a square wave signal, to each trace for charging/discharging a capacitor on each trace, and converts a voltage signal on each trace, which shows a charging/discharging curve, into a digital signal outputted to the microprocessor 102. When capacitance change happens on a trace, a voltage signal on the trace changes compared to a previous voltage signal taken on the same trace before the capacitance changed, so that a corresponding digital signal is also different from a previous digital signal taken before the capacitance changed. The microprocessor 102 is utilized for turning the A/D converter 100 on and off, for controlling the A/D converter 100 to scan the touch panel 12, and for adjusting operation modes of the touch panel 12 according to the digital signal generated by the A/D converter 100, so that the touch panel 12 can be operated within acceptable current consumption parameters. Briefly, the touch control device 10 determines that a touch event happens according to capacitance change on a trace, and controls operation modes of the touch panel 12.
When the touch control device 10 is scanning a trace, if other traces which are not scanned are floating and are not at a fixed voltage level, a human body capacitor may be coupled to the scanned trace, and a location of this touch event may not be detected correctly when a user touches the touch panel 12. Therefore, traces which are not scanned are usually kept at a fixed voltage level, such as a ground voltage level. When a trace in the touch panel 12 is touched, a capacitance on the touched trace increases and is different from a previous capacitance on the same trace before it was touched. Note that different capacitances correspond to different charge/discharge characteristic curves, so that different digital signals are generated. Therefore, the microprocessor 10 determines the touch event happens according to the digital signal.
The touch control device 10 operates in a drive mode or a sleep mode. During the drive mode, the touch control device 10 sequentially scans each trace in the touch panel 12, i.e. the A/D converter 100 sequentially outputs the charge control signal to every trace, for detecting any touch event. When the touch panel 12 consists of 20 traces, for example, the touch control device 10 takes 20 cycles to complete scanning for the whole touch panel 12. When size of the touch panel 12 increases, the number of traces in the touch panel 12 increases accordingly, and the touch control device 10 has to take more time to complete scanning. During the sleep mode, the touch control device 10 stops scanning. For general use, the time that the touch panel 12 is in use is only a small part of the day; when the human body leaves the touch panel 12 for a predetermined time, the touch control device 10 decreases the period of scanning; in other words, the touch control device 10 operates in the drive mode and in the sleep mode in turns in order to reduce current consumption.
Please refer to FIG. 2, which is a timing diagram of the touch control device 10. When a human body continuously touches the touch panel 12, the touch control device 10 operates in the drive mode, and the touch panel 12 operates in a normal mode. In this situation, the total average current consumption of the touch control device 10 and the touch panel 12 is 5 mA or so. After the human body leaves the touch panel 12, the touch control device 10 still operates in the drive mode for 2 more seconds, to be ready for the human body to touch the touch panel 12 again soon. When the touch control device 10 does not detect any touch events within the 2-second period, the touch control device 10 engages a periodic scheme by which the drive mode and the sleep mode are operated in turns. As shown in FIG. 2, the touch control device 10 initially operates with a 32 ms sleep mode and an 8 ms drive mode in turns, with average current consumption of around 1 mA, whereas a data report rate decreases from 136 Hz to 20 Hz. After the periodic scheme cycles through the 32 ms sleep mode and the 8 ms drive mode for 10 seconds, the touch panel 12 enters a doze mode, in which the touch control device 10 further decreases use of the drive mode, changing the periodic scheme to cycle a 152 ms sleep mode and the 8 ms drive mode to further reduce current consumption. In this situation, the current consumption does not exceed 250 uA, and the data report rate decreases from 25 Hz to 6.25 Hz. After the periodic scheme cycles through the 152 ms sleep mode and the 8 ms drive mode for 60 seconds, the touch control device 10 enters the sleep mode and does not do any scanning; the touch panel 12 also enters the sleep mode. In this situation, the current consumption does not exceed 50 uA, and the data report rate decreases to its lowest rate of 0 Hz. Under the sleep mode, the user cannot wake up the touch control device 10 through touch; the touch control device 10 can only be woken up through an external interrupt signal.
From the above, the touch control device for controlling the projected capacitive touch panel has around 5 mA current consumption in the drive mode. For optimizing current consumption, the touch control device has to operate in the periodic scheme, in which the sleep mode and the drive mode are operated in turns. Note that, during operation in the periodic scheme, performance of the data report rate in the touch control device is diminished, such that the touch control device 10 cannot detect touch events sensitively. Further, as number of traces increases, the touch control device has to take more time to complete scanning for the whole touch panel.